ARM: dts: imx6-tx6: add a .dtsi file for the MB7 baseboard

Move the MB7 specific settings to a separate .dtsi file to facilitate
supporting more module variants with this baseboard.

Signed-off-by: Lothar Waßmann <LW@KARO-electronics.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>